1. Field of the Invention
The present invention relates to a trace display apparatus for a navigation system which is mounted on a mobile body.
2. Description of the Related Art
Generally, a conventional navigation system mounted on an automobile employs a positioning system in which the dead reckoning, the map-matching, and the global positioning system (hereafter, abbreviated as "GPS") are used in combination. In the dead reckoning, the traveling locus of the automobile is obtained by means of an azimuth sensor and a distance sensor which are mounted on the automobile, and a relative position of the automobile is calculated. A prior art of navigation system using the dead reckoning is disclosed, for example, in Japanese Unexamined Utility model Publication (Kokai) No. JP-U 57-32,980 (1982). In the map-matching, the position of the automobile on a map results from comparing the traveling locus of the automobile obtained by the dead reckoning with road arrangement appearing on the map, so as to determine the road on which the automobile has passed. Since the map-matching determines a relative position of the automobile, errors may be accumulated in the position data. However, when the position data are combined with the information about the absolute position of the automobile which is obtained from the GPS, the resulting position data have a higher accuracy.
The GPS is a novel positioning system which has been developed by the National Military Establishment of the U.S. and uses a plurality of artificial satellites. The artificial satellites are orbiting around the earth and always transmitting signals indicative of orbit data, the time or the like. At a measuring point, signals transmitted from four satellites among these artificial satellites are received, and the time periods required for arrival of the signals are obtained. The distance between each artificial satellite and the measuring point is calculated from these time periods, so that it is possible to obtain the three-dimensional position (latitude, longitude and altitude) of the measuring point. When the two-dimensional position (latitude and longitude) of the measuring point is to be obtained, signals from three artificial satellites are received to calculate. In the shade of buildings in an urban area or in a tunnel, however, it is impossible to receive signals from the artificial satellites. In such a case, the position of an automobile is obtained by the dead reckoning. Japanese Unexamined Patent Publication (Kokai) No. JP-A 2-243,984 (1990) discloses a prior art system in which, using artificial satellites, the position (latitude and longitude) of a measuring apparatus on the earth is obtained in a manner different from that of the GPS.
When the position of an automobile is measured in a navigation system, a map including the automobile position is displayed on screen of a display apparatus. The information of the map is previously stored in a read-only memory using a compact disk for the navigation system (hereafter, such a memory is abbreviated as "CD-ROM"). When necessary information is read out from the CD-ROM, a mark indicating the automobile position is then superimposed on the displayed map. When the automobile position almost reaches one end of the map, the display is automatically changed to the adjacent map. Furthermore, such system can receive transportation information from a road beacon to display it on the screen. The system can receive also position information to correct the obtained current position.
FIG. 6 shows an example of the display of moving traces 1 of an automobile as a traveling locus on a display apparatus by a typical prior art navigation system. The automobile departs from the starting point 2. The two-dimensional position (latitude and longitude) of the automobile is obtained by a position measurement using the GPS or the like, and the positions are recorded until the automobile arrives the end point 3. In the figure, the thin solid lines indicate roads on the map, and the thick solid lines indicate the moving traces 1 of the automobile.
FIG. 7 shows an example of the display to indicate moving traces 5 and an altitude for an automobile on a display apparatus according to another prior art navigation system. In FIG. 6, the moving traces 1 of the automobile are only two-dimensionally displayed. In FIG. 7, when the user designates a point 6 on the screen, the altitude value 7 corresponding to the point 6 is displayed in the right and lower portion of the screen. Prior art navigation systems using the GPS are disclosed, for example, in Japanese Unexamined Patent Publications (Kokai) No. JP-A 63-26,529 (1988) and JP-A 3-17,688 (1991).
A trace display apparatus for a navigation system according to any prior art, the altitude of a mobile body is not displayed as shown in FIG. 6, or the altitude value of an arbitrary point designated by the user is displayed on a screen. Therefore, it is not easy to grasp the altitude change in the traces of a mobile body from a starting point to an end point.